Solid polymer electrolytes have recently attracted attention as electrolytes for lithium cells and the like because of the following advantages: (1) the energy density of a cell can be increased because the material can double as a separator, (2) leakage-free, high-reliability cells can be obtained by providing an all-solid construction, (3) it is easier to reduce the thickness or weight of a cell or to obtain an irregular shape, and the like.
There are two types of conventional solid polymer electrolytes: (1) polymers containing metal salts and (2) polymeric gels containing electrolyte solutions. With the first type, complexes of metal salts and polar polymers such as PEO (polyethylene oxide) form, and transport of lithium and other such ions accompanies the molecular motion of polymer chains. Such solid polymer electrolytes have high mechanical strength, but their ionic conductivity at room temperature has a limit on the order of 10−4 S/cm. It is therefore necessary to lower the molecular weight or to soften the polymers in order to intensify the molecular motion of the polymer chains, but this approach ultimately leads to a reduction in mechanical strength. With the second type, the contained electrolyte functions as an ionic conductor and preserves the polymers as solids. The ionic conductivity of such solid polymer electrolytes is on the order of 10−3 S/cm, that is, falls within a practicable range, but a disadvantage is that the polymers are plasticized by the electrolyte, and their mechanical strength is lowered.
Demand has existed for some time for solid polymer electrolytes whose ionic conductivity is on the order of 10−3 S/cm, whose thickness is on a par with that of conventional separators, and which have strength that does not present problems in terms of handling. Heat resistance is another consideration that has come into play in recent years as the performance of electrochemical reaction apparatuses has improved. In other words, a solid polymeric electrolyte composite for an electrochemical reaction apparatus should be able to preserve its diaphragm functions even when the apparatus heats up.
Composite solid polymeric electrolytes obtained by packing a solid polymer electrolyte into the pores of a polymeric porous film have been proposed as products satisfying both the ionic conductivity and mechanical strength requirements for solid polymer electrolytes (Japanese Laid-Open Patent Applications 1-158051, 2-230662, and 2-291607), but a satisfactory electrolyte has yet to be obtained.
Therefore, one object of the present invention is to provide a composite that utilizes a solid polymeric electrolyte for an electrochemical reaction apparatus that possesses satisfactory ion conduction properties and has excellent mechanical strength and heat resistance, and to provide an electrochemical reaction apparatus in which this electrolyte is used.
Ion exchange membranes are well known. Ion exchange membranes which utilize a microporous media have previously been disclosed (U.S. Pat. Nos. 5,547,551 and 5,599,614). Hitherto, the use of a microporous media was proposed primarily as a means of providing a “mechanical reinforcement function” only of the ion exchange media. This mechanical reinforcement provided improved dimensional stability as well as the capability to provide thinner overall membranes which in turn improved overall transport properties of the film (as measured through ionic conductance or moisture vapor transmission).
Also attempts to enhance ion exchange membrane properties have been attempted in the past by adding an additional component. U.S. Pat. No. 5,547,911 to Grot relates to a method to apply a layer of catalytically active particles to the surface of a membrane. U.S. Pat. No. 4,568,441 relates to the application of non-conductive inorganic particle to the surface of a membrane to improve gas release properties. Neither of these teaches that the dispersion of an additive within the membrane results in higher performance.
U.S. Pat. No. 5,322,602 to Razaq relates to improving the performance of an ion exchange polymer membrane by treating it with an acid which diffuses into the membrane.
WO 96/29752 to Grot et al relates to the incorporation of various inorganic fillers into a membrane to decrease fuel crossover. The ability to make thin very high conductance membranes is not addressed.
U.S. Pat. No. 5,523,181 (and Japanese patents 6-111827 and 6-111834) to Stonehart et al relates to an ion exchange membrane in which silica is dispersed throughout the membrane. No indication is made to a microporous substrate.
U.S. Pat. No. 5,472,799 to Watanabe relates to an ion exchange membrane which incorporates a catalyst layer. While a thin membrane is mentioned as desirable, no mention is made of a microporous support.
U.S. Pat. Nos. 5,547,551 and 5,599,614 relate to the use of a microporous support where the function is to improve strength and mechanical properties, allowing the use of thin high conductance membranes. The addition of fillers within the microporous support is not addressed; however, the use of additives with the ion exchange medium to enhance specific functional properties is disclosed. But it is difficult to distribute additive particles adequately since the microporous reinforcement also acts as a filtration medium impeding the flow of finely divided particulates.
There remains a need for thin high conductance membranes which have enhanced properties through the use of a functional support with the capability to provide multiple functions uniformly.